In recent years, a thin film transistor (hereinafter, a TFT has been developed. The TFT which uses a polycrystalline silicon film (polysilicon film) as a crystalline semiconductor film has been attracted attention. Especially, in a liquid crystal display deice (liquid crystal display) or and EL (electro-luminescence) display device (EL display), the TFT is used as an element for switching pixels and for forming a driver circuit which controls the pixels.
A polycrystalline silicon film is used as an active layer for a thin film transistor. A source region, a drain region, and a channel formation region are formed by doping impurity elements belonging to 13th group or 15th group in the periodic table. An offset region or an LDD region may be provided.
As a means for obtaining a polycrystalline silicon film, a technique of forming a polycrystalline silicon film by crystallizing an amorphous silicon film is generally used. Especially, a method for crystallizing an amorphous silicon film using laser light is attracted attention in recent years. As used herein, the term “laser crystallization” refers to a means for obtaining a polycrystalline semiconductor film by crystallizing or recrystallizing a semiconductor film by laser light.
After doping impurity elements to a polycrystalline silicon film for forming a source region, a drain region, and an LDD region, the value of resistance is reduced by heating the polycrystalline silicon film. The process is referred to as activation. As used herein, the term “laser activation” refers to activation using laser light.
As used herein, the term “laser annealing” refers to laser light irradiation to these semiconductor films for heating.
By laser annealing, a semiconductor film can be heated instantly. Therefore, laser annealing is effective as a means of annealing a semiconductor film formed over a substrate having low heat resistance such as a glass substrate or a plastic substrate.